This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, this invention relates to inductively coupled multi-section radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals.
In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.0, a greater number of nuclear-magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.0, also referred to as the polarizing field, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strenth of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the equation .omega.=.gamma.B, in which .gamma. is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B.sub.0 plus other fields) acting upon the nuclear spin. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.0, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonal to the direction of magnetization M by means of radio-frequency pulses through a coil connected to radio-frequency-transmitting apparatus. Magnetization M rotates about the direction of the B.sub.1 field. In NMR, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B.sub.0 field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane begin to realign with the B.sub.0 field by a variety of physical processes. During this realignment process, the nuclear moments emit radio-frequency signals, termed the NMR signals, which are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. The same or a second RF coil may be used to receive the signals emitted from the nuclei. In NMR imaging applications, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the NMR signal. This information is later used to reconstruct images of the object studied in a manner well known to those skilled in the art.
The use of a solenoidal geometry has been found advantageous in the design of magnets used to produce the B.sub.0 magnetic field. The use of this geometry, however, imposes two constraints on the design of RF coils. One of these constraints it that the radio-frequency field B.sub.1 produced by the RF coil must be perpendicular to the solenoidal axis of symmetry which is parallel to the axis of field B.sub.0. The other constraint is that the RF coil should be constructed on the surface of a cylinder to provide free access along the solenoidal axis to accommodate the patient to be examined. Conventional RF coils are typically constructed on a cylindrical form which has the resonant circuit mounted on the outside surface.
In practice, the use of a cylindrical RF coil geometry requires that the patient be positioned in the active coil region. One way in which this may be achieved is to position a patient on a cradle transport mechanism and to then transport the patient longitudinally into the coil which would be mounted concentrically in the bore of the magnet. Another way in which this may be accomplished, as in the case of NMR head studies, is to place the coil around the head and then position both in the B.sub.0 magnetic field. There are a number of disadvantages associated with the conventional coil design. For example, in the case of head studies, the single-piece cylindrical construction limits the operator's view of the patient. This may make it difficult for the operator to monitor the condition of the patient. Such construction may also make it more difficult to position the patient within the coil to obtain optimum results. In some cases, it is desirable to obtain both head and body images utilizing separate RF coils for each. To minimize the data-collection time, it is expeditious to leave the head coil in place while only the body coil is energized. This, however, creates a problem in that there is undesired coupling between the two coils which could affect image quality. It is, therefore, a principal object of the present invention to provide an improved NMR RF coil construction which overcomes the aforementioned disadvantages.